p0f v2 is a passive operating system fingerprinting tool.  
It can identify the OS on:
   - machines that connect to your box (SYN mode),
   - machines you connect to (SYN+ACK mode),
   - machines you cannot connect to (RST+ mode),
   - machines whose communications you can observe.

P0f can also detect or measure the following:
   - firewall presence, NAT use,
   - existence of a load balancer setup,
   - the distance to the remote system and its uptime,
   - other guy's network hookup (DSL, OC3, avian carriers) and his ISP.
